The Modern Student laboratory Potentiometric Titrotions
Determination of the Hole Concentration (Copper Valency) in the High Tc Superconductors A Laboratory Using the Potentiometric Titration Method P. Phinyocheep Faculty of Science, Mahidol University, Rama VI Road, Bangkok 10400,Thailand I. M. Tang
Department of Physics, Faculty of Science, Mahidol University, Rama VI Road, Bangkok 10400. Thailand The single most important factor for achieving superconductivity in the perovskite structure cuprate oxide ceramics (1)is the hole concentration in the two-dimensional CuOz layer (2).At first, it was thought that the oxygen wntent was the controlling factor. However, achievement (3) of the optimal T, in oxygen-vacancy-free LakSr,Cu04 through the nonisovalent S?+/La3+ substitution clearly demonstrated that it was the hole concentration that was important. Shortly after the start of worldwide interest in the field, an article appeared in this Journal (41, giving the details of the iodometric titration method for determining the copper oxidation states in a 90 K superconducting ceramic YBazCu307, specimen. In the wurse of studying the superconducting properties we of the bismuth su~erconductor(Bi.Pb)oSr9CaCuo0.-,. . . " encountered some difficulties in using the iodometric titration method to determine the valencies of the comer ions (5).The change in wlor of the solution, from be&;ning to endpoint of the titration, was from reddish brown to just brown and was difficult to see. We believe that the second brown is due to the presence of bismuth iodate in the solution. To determine the endpoint of the titration, we have used the potentiometric titration method, which we will describe in this short note. " , J ,
The number of moles of thiosulfate used to titrate the iodine is equal to the number of moles of Cu ions (both the divalent and trivalent species). In the second part, an accurately weighed amount of the superconductor is dissolved in a mixture of 1.0 M HC1 and 0.7 M KI. The Cu3+ionsare not reduced to Cu2+before adding KI in this part of the experiment. The different Cu ions react as follows.
and
The number of moles of thiosulfate needed to titrate the liberated I2 would now corrcspond to 1 mol Cu2'and 2 mol Cu3'. The difference between the results of the two onrts of the titration is the content of the Cu3+in the specim'ens.All
Review of the lodometric Titration Method 'Caution:All steps involving the boiling of acid solutions should be done in a fume hood. As pointed out in ref 4, the number of moles of Cu ions (both Cu3+and Cu2') is first determined. Dissolve an accurately weighed amount ofYBa2Cus07, in 1.0 M HC1, and then gently boil the solution to insure the destruction of all Cu3+.Water containing 1.0-1.5 g of KI is then added. The following reaction takes place.
Calomel electrode 1 M NHN . O,
Bar
The solution is then treated with iodide to get Magnetic
Stirrer
Titration of the liberated iodine with thiosulfate yields Figure 1. Schematic of experimental setup. (Continned on next page) A115
Volume 71 Number 5 May 1994
The Modern Student laboratory Potentiometric Titrotions solution in beaker Aduring the titration with freshly prepared NazSzOs solution. The potential differencebetween the two electrodes as the titratian is carried out is then recorded. The use ofcombincdmetal elenruden with a commarrial potentiometric titrator eliminates the need for the second beaker and the salt bridge. ~~~~~~~
-20
.
,
-30-1. 0
20
40
Experimental Run To demonstrate the need for potentiometry and equivalence of the results obtained by the two methods, we have determined the valencies of the Cu ions i n t h e superconductors DyBazCu307, and Bi1.6Pb0.4Sr2. C~O.~YO.ZCU~O~,. (These two compositions were used because they were already available in the laboratory.) Following the details given in ref 4, we prepared the following solutions.
. 60
110
VOLUME Iml)
-50 0
20
40
60
i
VOLUME (ml)
Solutions
0.03 M NazSz03 Solution: In 500 mL of freshly boiled distilled water dissolve 3.7 g of NazSz03~5Hz0a n d 0.05 g of Na2C03.
VOLUME (ml)
Figure 2. Potentiometric data for standardization of Na2S203with Cu. The main culve shows the potentiometric reading as the titration is carried out. The inserts show the first and second derivatives of the main curve. The endpoint is obtained from the second derivative culve. Standard Cu Solution: Accurately weighed amount of reagent Cu shots (O.&0.6 g) is added to 10 mL of distilled of the titration is to be done under a flowing nitrogen gas atmosphere. water and 3 mL of 70% nitric acid. This is then gently boiled to dissolve the solid Cu. Then add 10 mL of distilled water and 0.5 g of sulfamic acid. Boil to destroy HNOZand any other oxides of nitrogen that might interfere with the Modification to Potentiometric Determination of Endpoint titration. The solution is cooled to room temperature and then diluted to 100 mL with 1.0 M HCI. (All this is done in The above iodiometric titration works quite well with the a fume hood.) YBaEu2Cu7_.and other "123" suoerconductors. When aoplied to the'dktermination of the'cu valencies in the bi'smuth superconductors, the above iodometric analysis presStandardization of N&203 with Co ents some difficulties due to the formation of bismuth iodate in the solution, which is of a slightly different shade Add 10.00 mL of standard Cu to 10 mL of distilled water of brown. To overcome the difficulty of distinguishing the containing 1.0-1.5 gof KI (freshly dissolved). Carry out the different shades of brown. we have adouted the uotentiotitration with the Na2Sz03solution in a 50-mL buret, and metric titration method (fi). record the potential difference between the electrodes. The eeneral outline of the method can be found in Chao(See F i r e 2. The endpoint is determined from the second ter 14 i f ref 6. Details of the method (see Fig. 1)are derivative curve.) below. The treated solution (prepared in the same way as above) to be titrated is placed in beaker A, which contains a magnetic stirrer. AU-shape tube containing 1M NH4NOaacta as a salt bridge connecting heaker Ato second beaker B, which also contains 1M NHINOI in solution. A Pt electrode is immersed in the treated soiution in beaker A while a calomel electrode is ~mmersedin the 1 M NII,N03 ~ n l u t m nin henker B l?le two electrodes nn- connected to the rnputs of a dlk~talvoltmeter (capableofreadmg mV, A hnsk N, flow is mamwlned over th? All6
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Titration Once the NazszO~ is standardized, accuratelv weiehed - amounts of the two superconductors are disso6ed as described under Standard Cu Solution. The potentiometric titration is now carried out as described i n t h e standard~
(Continued on page A218)
The Modern Student laboratorv Potentiometric Titrntions
I 10
20
30
40
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VOLUME (ml)
Figure
3.
Potentiometric
data
for
titration
of
Bi,.6Pbo.4Sr,Cao,6Yo.20B+Y' No distinct color change was seen while passing the endpoint given by the second derivative of the curve.
Eventually, the brown color turned to a muddy gray.
ization step. Two drops of a clear starch solution were added just as the potentiometric drop began to aid in monitoring the end of the titration by the observation of the color &anges. I n step A of the titration of the DyBa2Cu30,, specimen, the color chanee o c m e d when 25.6-25.7 mL of the standardlzed t h ~ o s k a t esolution was added. Potentlometry Indlcated that the cndoolnt was 25.6 mL Firmrc Y shows the titration data for the Bil.sPbo.4SrzCao.8Yo.zCuz08, solution. No s t r o w color change was seen while as sing the endpoint (1538 mL as ditermined from t h l pote&ometry). The results of the titration show that the Cu valency in DyBa2Cu30,, was +2.23, w h e r e a s t h e valency i n Bil.6Pbo.4SrzCao.sYo.~C~& was +2.19 (7).Both values are in the ranges of copper valencies a t which optimal superconductivity is observed in these two s u ~ e r w n d u d o r s . Another Acthod for determining the hole concentration is first to determine the oxygen content hy TGA(thermogravimrtnc analysis, under a reducing atmosphere. 'l'hn ropper valency is then determined hy charge balancing, assuminrr " that the Bi and I'h ions are in the 3+ and I + states. respectively. However, Retoux et al. (8)have presented evidence that the bismuth may be in a lower valency state. The determination of the copper valency by charge balancing would then be in question.
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Literature Cited
1.Bednorz,J. C.:Mueller, K A Z Phya.B 1986, M , 189. 2. Whmgbo,M.H.;lbradi.C. CScienee 1980,249,1143. 3.lbmnce.J.B. etd.Phys.Rev.Left 1988,61,1127. 4.Hams.D.C.: Ells. M. E.:Hewston.TA. J Chom.Educ. 1987.847. 5. ~ a n g , M. a ~ . ~ h ybsi.t . A m u : MY. 311. 6. Vogel,A. I A Teubmk ofQuonliloliua Inorganic Analyaiz,3rd ed.:longnan: London, 1961; Chapter 14.
i. ei
7.Tang.I. M.;Leelepmte, 8.;Setsuwan,P Phyaico C 1981.177.57. 8.Retoux, R, et d.Phys. Re". B 1980,41,193.
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